U.S. patent number 5,861,219 [Application Number 08/838,099] was granted by the patent office on 1999-01-19 for organic light emitting devices containing a metal complex of 5-hydroxy-quinoxaline as a host material.
This patent grant is currently assigned to The Trustees of Princeton University, The University of Southern California. Invention is credited to Paul E. Burrows, Stephen R. Forrest, Andrei Shoustikov, Mark E. Thompson, Yujian You.
United States Patent |
5,861,219 |
Thompson , et al. |
January 19, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
Organic light emitting devices containing a metal complex of
5-hydroxy-quinoxaline as a host material
Abstract
The present invention is directed to organic light emitting
devices comprised of an electroluminescent layer containing a host
material comprised of a metal complex of (5-hydroxy)quinoxaline:
##STR1## wherein M is Al, Ga, In, Zn or Mg, with n=3, if M is Al,
Ga or In and n=2, if M is Zn or Mg, and to a method for fabricating
such devices. Further disclosed for use in the electroluminescent
layer of organic light emitting devices are dopant materials
comprised of a bisphenyl-squarilium compound, an indigo dye
compound or a fullerene compound.
Inventors: |
Thompson; Mark E. (Anaheim
Hills, CA), Forrest; Stephen R. (Princeton, NJ), Burrows;
Paul E. (Princeton, NJ), You; Yujian (Los Angeles,
CA), Shoustikov; Andrei (Los Angeles, CA) |
Assignee: |
The Trustees of Princeton
University (Princeton, NJ)
The University of Southern California (Los Angeles,
CA)
|
Family
ID: |
25276258 |
Appl.
No.: |
08/838,099 |
Filed: |
April 15, 1997 |
Current U.S.
Class: |
428/690; 428/691;
313/504; 428/917 |
Current CPC
Class: |
B82Y
10/00 (20130101); H01L 51/0092 (20130101); H01L
51/0082 (20130101); H01L 51/0079 (20130101); H01L
51/0081 (20130101); H01L 51/5206 (20130101); H01L
51/0053 (20130101); H01L 51/0046 (20130101); H01L
51/5012 (20130101); H01L 2251/5315 (20130101); Y10S
428/917 (20130101); H01L 2251/308 (20130101); H01L
51/5281 (20130101) |
Current International
Class: |
H01L
51/50 (20060101); H01L 51/52 (20060101); H01L
51/30 (20060101); H01L 51/05 (20060101); H05B
033/00 () |
Field of
Search: |
;428/690,691,917
;313/504 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Coord. Chem. Rev. (1998), 171, 161-174 May 1998. .
Synth. Met. (1997), 91 (1-3), 217-221 May 1997. .
Gatterer, A., "Spectrochimica ACTA," vol. 8, '956, pp. 1-8.
Pergamon Press Ltd., London. .
S. K. Freeman, P. E. Spoerri, Journal of Organic Chemistry, vol.
16, 1951, pp. 438-442. .
Forrest, S. et al., "Organic emitters promise a new generation of
displays," Laser Focus World, Feb. 1995, pp. 99-107. .
You, Y. et al., New Orange and Red Organic Light-Emitting Devices
(OLEDS) Using Aluminum Tris(5-Hydroxyquinoxaline), 213TH ACS
National Meeting, San Francisco, CA, Apr. 13-17, 1997, Poster
Session Paper No. 619, Apr. 15, 1997, (Abstract)..
|
Primary Examiner: Nold; Charles
Attorney, Agent or Firm: Kenyon & Kenyon
Government Interests
GOVERNMENT RIGHTS
This invention was made with Government support under Contract No.
F33615-94-1-1414 awarded by DARPA. The government has certain
rights in this invention.
Claims
What is claimed is:
1. An organic light emitting device for producing
electroluminescence comprising:
a heterostructure for producing electroluminescence, said
heterostructure having an emissive layer comprised of a host
material and a dopant, said host material being comprised of a
metal complex of (5-hydroxy)quinoxaline having the chemical
structure of formula I: ##STR12## wherein M is Al, Ga, In, Zn or
Mg, with n=3, if M is Al, Ga or In and n=2, if M is Zn or Mg.
2. The organic light emitting device of claim 1 wherein the metal
complex of (5-hydroxy)quinoxaline comprises an aluminum complex
having a chemical structure of formula II: ##STR13##
3. The organic light emitting device of claim 1 wherein said dopant
is comprised of a bisphenyl-squarilium compound of formula III:
##STR14## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are,
independently of each other, an unsubstituted or substituted alkyl,
aryl or heterocycle, and R.sub.5 and R.sub.6 are, independently of
each other, an unsubstituted or substituted alkyl, aryl, OH or
NH.sub.2.
4. The organic light emitting device of claim 1 wherein said dopant
comprises a squarilium dye having a chemical structure of formula
IV: ##STR15## wherein R=alkyl.
5. The organic light emitting device of claim 1 wherein said dopant
comprises the squarilium dye having the chemical structure:
##STR16##
6. The organic light emitting device of claim 1 wherein said dopant
comprises an indigo dye compound having a chemical structure of
formula: ##STR17## wherein X=NH, NR.sub.9, S, Se, Te, or O, wherein
R.sub.9 is alkyl or phenyl, and R.sub.7 and R.sub.8 are,
independently of each other, an unsubstituted or substituted alkyl
or aryl group, a .PI.-electron donor group or a .PI.-electron
acceptor group.
7. The organic light emitting device of claim 1 wherein said dopant
comprises an indigo dye compound having a chemical structure of:
##STR18## wherein X is NH.
8. The organic light emitting device of claim 1 wherein said dopant
comprises a fullerene compound.
9. The organic light emitting device of claim 8 wherein said
fullerene compound comprises C.sub.60.
10. A display incorporating the organic light emitting device of
claim 1.
11. A vehicle incorporating the organic light emitting device of
claim 1.
12. A computer incorporating the organic light emitting device of
claim 1.
13. A television incorporating the organic light emitting device of
claim 1.
14. A printer incorporating the organic light emitting device of
claim 1.
15. A wall, theater or stadium screen incorporating the organic
light emitting device of claim 1.
16. A billboard or a sign incorporating the organic light emitting
device of claim 1.
17. An organic light emitting device for producing
electroluminescence comprising:
a heterostructure for producing electroluminescence, said
heterostructure having an emissive layer comprised of a host
material and a dopant, said dopant being comprised of a compound of
formula III: ##STR19## wherein R.sub.1, R.sub.2, R.sub.3 and
R.sub.4 are, independently of each other, an unsubstituted or
substituted alkyl, aryl or heterocycle, and R.sub.5 and R.sub.6
are, independently of each other, an unsubstituted or substituted
alkyl, aryl, OH or NH.sub.2.
18. The organic light emitting device of claim 17 wherein said
dopant comprises a squarilium dye having a chemical structure of
formula IV: ##STR20## wherein R=alkyl.
19. The organic light emitting device of claim 17 wherein said
dopant comprises the squarilium dye having the chemical structure:
##STR21##
20. An organic light emitting device for producing
electroluminescence comprising:
a heterostructure for producing electroluminescence, said
heterostructure having an emissive layer comprised of a host
material and a dopant, said dopant being comprised of an indigo dye
compound of formula V: ##STR22## wherein X=NH, NR.sub.9, S, Se, Te,
or O, wherein R.sub.9 is alkyl or phenyl, and R.sub.7 and R.sub.8
are, independently of each other, an unsubstituted or substituted
alkyl or aryl group, a .PI.-electron donor group or a .PI.-electron
acceptor group.
21. The organic light emitting device of claim 20 wherein said
.PI.-electron donor group is --OR, --Br or --NR.sub.2.
22. The organic light emitting device of claim 20 wherein said
.pi.-electron acceptor group is --CN or --NO.sub.2.
23. The organic light emitting device of claim 20 wherein said
dopant comprises an indigo dye compound having a chemical structure
of formula VI: ##STR23## wherein X is NH.
24. An organic light emitting device for producing
electroluminescence comprising:
a heterostructure for producing electroluminescence, said
heterostructure having an emissive layer comprised of a host
material and a dopant, said dopant being comprised of a fullerene
compound.
25. The organic light emitting device of claim 24 wherein said
fullerene compound comprises C.sub.60.
26. A method of fabricating an organic light emitting device for
producing electroluminescence comprising:
fabricating a heterostructure for producing electroluminescence,
wherein the fabrication process includes the step of depositing a
layer comprised of a host material and a dopant, said host material
being comprised of a metal complex of (5-hydroxy)quinoxaline having
the chemical structure of formula I: ##STR24## wherein M is Al, Ga,
In, Zn or Mg, with n=3, if M is Al, Ga or In and n=2, if M is Zn or
Mg.
27. The method according to claim 26 wherein the metal complex of
(5-hydroxy)quinoxaline comprises an aluminum complex having a
chemical structure of formula II: ##STR25##
28. The method according to claim 26 wherein said dopant is
comprised of a compound of formula III: ##STR26## wherein R.sub.1,
R.sub.2, R.sub.3 and R.sub.4 are, independently of each other, an
unsubstituted or substituted alkyl, aryl or heterocycle, and
R.sub.5 and R.sub.6 are, independently of each other, an
unsubstituted or substituted alkyl, aryl, OH or NH.sub.2.
29. The method according to claim 26 wherein said dopant comprises
a squarilium dye having a chemical structure of formula IV:
##STR27## wherein R=alkyl.
30. The method according to claim 26 wherein said dopant comprises
the squarilium dye having the chemical structure of formula IVa:
##STR28##
31. A method of fabricating an organic light emitting device for
producing electroluminescence comprising:
fabricating a heterostructure for producing electroluminescence,
wherein the fabrication process includes the step of depositing a
layer comprised of a host material and a dopant, said dopant being
comprised of a compound of formula III: ##STR29## wherein R.sub.1,
R.sub.2, R.sub.3 and R.sub.4 are, independently of each other, an
unsubstituted or substituted alkyl, aryl or heterocycle, and
R.sub.5 and R.sub.6 are, independently of each other, an
unsubstituted or substituted alkyl, aryl, OH or NH.sub.2.
32. The method according to claim 31 wherein said dopant comprises
a squarilium dye having a chemical structure of formula IV:
##STR30## wherein R=alkyl.
33. The method according to claim 31 wherein said dopant comprises
the squarilium dye having the chemical structure of formula IVa:
##STR31##
34. A method of fabricating an organic light emitting device for
producing electroluminescence comprising:
fabricating a heterostructure for producing electroluminescence,
wherein the fabrication process includes the step of depositing a
layer comprised of a host material and a dopant, said dopant being
comprised of a indigo dye compound of formula V: ##STR32## wherein
X=NH, NR.sub.9, S, Se, Te, or O, wherein R.sub.9 is alkyl or
phenyl, and R.sub.7 and R.sub.8 are, independently of each other,
an unsubstituted or substituted alkyl or aryl group, a
.PI.-electron donor group or a .PI.-electron acceptor group.
35. The method according to claim 34 wherein said .PI.-electron
donor group is --OR, --Br or --NR.sub.2.
36. The method according to claim 34 wherein said .pi.-electron
acceptor group is --CN or --NO.sub.2.
37. The method according to claim 34 wherein said dopant comprises
an indigo dye compound having a chemical structure of formula VI:
##STR33## wherein X is NH.
38. A method of fabricating an organic light emitting device for
producing electroluminescence comprising:
fabricating a heterostructure for producing electroluminescence,
wherein the fabrication process includes the step of depositing a
layer comprised of a host material and a dopant, said dopant being
comprised of a fullerene compound.
39. The method according to claim 38 wherein said fullerene
compound comprises C.sub.60.
Description
FIELD OF THE INVENTION
The present invention is directed to organic light emitting devices
comprised of an electroluminescent layer containing a host material
comprised of a metal complex of (5-hydroxy)quinoxaline.
BACKGROUND OF THE INVENTION
Organic light emitting devices (OLEDs) are light emitting devices
that are comprised of several layers, in which one of the layers is
comprised of an organic material that can be made to
electroluminesce by applying a voltage across the device. Certain
OLEDs have been shown to have sufficient brightness, range of color
and operating lifetimes for use as a practical alternative
technology to LCD-based full color flat-panel displays (S. R.
Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World,
February 1995). Furthermore, since many of the organic thin films
used in such devices are transparent in the visible spectral
region, they allow for the realization of a completely new type of
display pixel in which the red (R), green (G), and blue (B)
emission layers are placed in a vertically stacked geometry to
provide a simple fabrication process, a small R-G-B pixel size, and
a large fill factor.
A transparent OLED (TOLED) which represents a significant step
toward realizing high resolution, independently addressable stacked
R-G-B pixels has been reported in International Patent Application
No. PCT/US95/15790. This TOLED had greater than 71% transparency
when turned off and emitted light from both top and bottom device
surfaces with high efficiency (approaching 1% quantum efficiency)
when the device was turned on. The TOLED used transparent indium
tin oxide (ITO) as the hole-injecting electrode and a Mg-Ag-ITO
layer for electron-injection. A device was disclosed in which the
Mg-Ag-ITO electrode was used as a hole-injecting contact for a
second, different color-emitting OLED stacked on top of the TOLED.
Each device in the stacked OLED (SOLED) was independently
addressable and emitted its own characteristic color through the
transparent organic layers, the transparent contacts and the glass
substrate, allowing the device to emit any combination of color
that could be produced by varying the relative output of the red
and blue color-emitting layers.
Thus, publication of PCT/US95/15790 provided the disclosure of an
integrated OLED where both intensity and color could be
independently varied and controlled with external power supplies in
a color tunable display device. As such, PCT/US95/15790 illustrates
a principle for achieving integrated, full color pixels that
provide high image resolution, which is made possible by the
compact pixel size. Furthermore, relatively low cost fabrication
techniques, as compared with prior art methods, may be utilized for
making such devices.
Devices whose structure is based upon the use of layers of organic
optoelectronic materials generally rely on a common mechanism
leading to optical emission. Typically, this mechanism is based
upon the radiative recombination of a trapped charge. Specifically,
devices constructed along the lines discussed above comprise at
least two thin organic layers separating the anode and cathode of
the device. The material of one of these layers is specifically
chosen based on the material's ability to transport holes (the
"hole transporting layer"); the other, according to its ability to
transport electrons (the "electron transporting layer"). The
electron transporting layer typically comprises the
electroluminescent layer. With such a construction, the device can
be viewed as a diode with a forward bias when the potential applied
to the anode is higher than the potential applied to the cathode.
Under these bias conditions, the anode injects holes (positive
charge carriers) into the hole transporting layer, while the
cathode injects electrons into the electron transporting layer. The
portion of the luminescent medium adjacent to the anode thus forms
a hole injecting and transporting zone while the portion of the
luminescent medium adjacent to the cathode forms an electron
injecting and transporting zone. The injected holes and electrons
each migrate toward the oppositely charged electrode. When an
electron and hole localize on the same molecule, a Frenkel exciton
is formed. Recombination of this short-lived state may be
visualized as an electron dropping from its conduction potential to
a valence band, with relaxation occurring, under certain
conditions, preferentially via a photoemissive mechanism. Under
this view of the mechanism of operation of typical thin-layer
organic devices, the electroluminescent layer comprises a
luminescence zone receiving mobile charge carriers (electrons and
holes) from each electrode.
It would be desirable if each of the color-emitting OLEDs that are
used in a SOLED could have the electroluminescent emission
efficiently produced in a relatively narrow band centered near
selected spectral regions, which correspond to one of the three
primary colors, red, green and blue. The materials that are
responsible for producing the electroluminescent emission are
frequently incorporated into the OLED such that they also serve as
the electron transporting layer of the OLED. Such devices are
referred to as having a single heterostructure. Alternatively, the
electroluminescent material may be present in a separate emissive
layer between the hole transporting layer and the electron
transporting layer in what is referred to as a double
heterostructure.
In addition to having the emissive material present as the primary
material in the electron transporting layer, the emissive material
may also be present as a dopant that is contained within a host
material. Materials that are present as host and dopant are
selected so as to have a high level of energy transfer between the
host and dopant materials. In addition, these materials need to be
capable of producing acceptable electrical properties for the OLED.
Furthermore, such host and dopant materials are preferably capable
of being incorporated into the OLED using starting materials that
can be readily incorporated into the OLED using convenient
fabrication techniques.
SUMMARY OF THE INVENTION
The present invention is directed toward host and dopant materials
that may be used in the electroluminescent layer of an OLED.
More specifically, the present invention is directed to host
materials that may be used to contain dopants selected to produce
electroluminescence in a relatively narrow band centered near the
saturation wavelength of one of the primary colors.
Still more specifically, the present invention is directed to host
and dopant materials that can be used to fabricate OLEDs that
produce predominantly red electroluminescence.
One of the features of the present invention is that the
color-emitting OLEDs may be especially effective for use in high
resolution, independently addressable stacked R-G-B pixels that are
incorporated into color tunable display devices.
Yet more specifically, the present invention is directed to a host
material in the electroluminescent layer of an OLED which is
comprised of a metal complex of 5-hydroxy-quinoxaline of formula I:
##STR2## wherein M is Al, Ga, In, Zn or Mg, with n=3, if M is Al,
Ga or In and n=2, if M is Zn or Mg.
Yet more specifically, the present invention is directed to a host
material comprised of an aluminumtris(5-hydroxyquinoxaline) having
the chemical structure of formula II: ##STR3##
In addition, one of the embodiments of the present invention is
directed to the inner salts of fluorescent dyes that may be used as
the dopant material in the electroluminescent layer of an OLED.
More specifically, one of the embodiments of the present invention
is directed to inner salts of bisphenyl-squarilium dyes that may be
used as the dopant material in the electroluminescent layer of an
OLED.
Still another of the embodiments of the present invention is
directed to indigo dye compounds that may be used as the dopant
material in the electroluminescent layer of an OLED.
Yet another of the embodiments of the present invention is directed
to fullerene compounds, for example, C.sub.60, as the dopant
material in the electroluminescent layer of an OLED.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a representative OLED for a single heterostructural
device.
FIG. 2 shows the photoluminescence (PL) spectra of
aluminumtris(5-hydroxy-quinoxaline),
aluminumtris(5-hydroxyquinoline) and gallium
bis(5-hydroxy-quinoxaline).
FIG. 3 shows the electroluminescence spectra of OLEDs containing an
emissive layer of aluminumtris(5-hydroxyquinoxaline) with and
without the inner salt of a bisphenyl-squarilium compound of
formula III as a dopant.
FIG. 4 shows the I-V characteristics of
aluminumtris(5hydroxy-quinoxaline) with and without the inner salt
of a bisphenyl-squarilium compound of formula III as the
dopant.
FIG. 5 shows the photoluminescent spectra of the host compounds
Alq.sub.3 and Alx.sub.3 as compared with the absorbance spectra of
the dopants: the inner salt of a bisphenyl-squarilium dye
("BIS-OH") an indigo dye compound and a fullerene compound,
C.sub.60.
FIG. 6 shows the photoluminescent spectra of the dopant compounds
in solution, the inner salt of a bisphenyl-squarilium dye
("BIS-OH") (in CH.sub.2 CI.sub.2), an indigo dye compound (in DMSO)
and C.sub.60 (in toluene).
FIG. 7 shows the electroluminescent spectra of TPD-Alq.sub.3
/C.sub.60 devices as a function of increasing C.sub.60
concentration in the host Alq.sub.3 material.
FIG. 8 shows the electroluminescent spectra of TPD-Alq.sub.3 /(the
bisphenol-squarilium dye of formula IVa) devices as a function of
the bisphenol-squarilium dopant concentration in the Alq.sub.3 host
material.
FIG. 9 shows the electroluminescent spectra of TPD-Alx.sub.3 /(the
bisphenol-squarilium dye of formula IVa) devices as a function of
the bisphenol-squarilium dopant concentration in the host Alx.sub.3
material.
FIG. 10 shows the electroluminescent spectrum of the TPD-Alx.sub.3
/indigo dye compound device with a 1.7% indigo dye compound
concentration.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention will now be described in detail for specific
preferred embodiments of the invention, it being understood that
these embodiments are intended only as illustrative examples and
the invention is not to be limited thereto.
The present invention is directed to substantially any type of OLED
structure in which the disclosed host and/or dopant materials may
be included in the emissive layer of a heterostructure that is used
for producing electroluminescence. As used herein, the term
"heterostructure for producing electroluminescence" refers to a
heterostructure that includes, for a single heterostructure such as
shown in FIG. 1, a hole-injecting anode layer 2, a hole
transporting layer 3 in contact with the anode layer, an electron
transporting layer 4 in contact with the hole transporting layer
and an electron-injecting cathode layer 5 in contact with the
electron transporting layer. Alternatively, if a double
heterostructure is used to produce electroluminescence, a separate
emissive layer (not shown in FIG. 1) is included between the hole
transporting layer and the electron transporting layer. One of the
electrodes of the heterostructure is in contact with a substrate 1.
Although FIG. 1 shows the anode to be in contact with the
substrate, the cathode may, instead, be in contact with the
substrate. Each electrode is in contact with a power supply for
providing voltage across the heterostructure. In addition, when the
heterostructure is in a stacked OLED (SOLED), one or both of the
electrodes of an individual heterostructure may be in contact with
an electrode of an adjacent heterostructure. Electroluminescence is
produced by the emissive layer of the heterostructure when a
voltage of proper polarity is applied across the heterostructure.
This electroluminescence is understood to be produced by the
recombination of holes and electrons in the electron transporting
layer of a single heterostructure and in the separate emissive
layer of a double heterostructure. In addition to the above-noted
layers, additional layers may also be present in the
heterostructure, for example, to protect the cathode layer from
oxidation or to protect a just deposited layer from damage that may
be caused during deposition of the next layer. For example, the
OLED may include a protection layer between the hole transporting
layer and the ITO. This protection layer may be formed by the
deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),
bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT), copper
phthalocyanine (CuPu) or other suitable, more rigid organic
materials.
The single or double heterostructures, as referred to herein, are
intended solely as examples for showing how an OLED embodying the
present invention may be fabricated without in any way intending
the invention to be limited to the particular sequence or order of
making the layers shown. For example, a single heterostructure
typically includes a substrate which may be opaque or transparent,
rigid or flexible, and/or plastic, metal or glass; a first
electrode, which is typically a high work function, hole-injecting
metal anode layer, for example, an indium tin oxide (ITO) anode
layer; a hole transporting layer; an electron transporting layer;
and a second electrode layer, for example, a low work function,
electron-injecting, metal cathode layer of a magnesium-silver
alloy, (Mg:Ag). Dependent on whether the (Mg:Ag) cathode layer
serves as the top electrode of the device, in which case, it may be
covered with an opaque protective metal layer, for example, a layer
of Ag for protecting the cathode layer from atmospheric oxidation;
or on whether the (Mg:Ag) cathode layer serves as a substantially
transparent top layer of a TOLED, in which case, the (Mg:Ag)
cathode layer is covered with a relatively thin layer of
substantially transparent ITO. In a TOLED, the relatively thin
substantially transparent ITO layer may function as the electrical
contact layer of the TOLED in addition to functioning as the
hole-injecting anode layer of the adjacent OLED layer.
When the OLED is comprised of a double heterostructure having an
additional layer of emissive material between the hole transporting
and electron transporting layers, this additional layer may be
referred to as a "separate emissive layer" so as to distinguish it
from the electron transporting layer of a single heterostructure
that functions both as the electron transporting layer as well as
the emissive layer that produces the electroluminescence. The term
"emissive layer" as used herein, thus, may refer either to the
emissive, electron transporting layer of a single heterostructure
or the separate emissive layer of a double heterostructure.
Inverted OLEDs (IOLEDs) are devices in which the order of placing
the sequence of layers onto the substrate is reversed. In
particular, for a single heterostructure, the electron-injecting
cathode layer is deposited onto the substrate, the electron
transporting layer is deposited on the cathode, the hole
transporting layer is deposited on the electron transporting layer
and the hole-injecting anode layer is deposited on the hole
transporting layer.
Although not limited to the thickness ranges recited herein, the
substrate (element 1 in FIG. 1) may be as thin as 10.mu., if
present as a flexible plastic or metal foil substrate, or
substantially thicker if present as a rigid, transparent or opaque,
substrate; the ITO anode layer may be about 500 .ANG.
(1.ANG.=10.sup.-8 cm) to greater than about 4000 .ANG. thick; the
hole transporting layer about 50 .ANG. to greater than about 1000
.ANG. thick; the separate emissive layer of a double
heterostructure about 50 .ANG. to about 200 .ANG. thick; the
electron transporting layer about 50 .ANG. to about 1000 .ANG.
thick; and the metal cathode layer about 50 .ANG. to greater than
about 100 .ANG. thick, or substantially thicker if the cathode
layer includes a protective silver layer and is opaque. The PTCDA
protection layer, if present, may be from about 50 .ANG. to about
500 .ANG. thick.
The emission of red light in light emitting devices has been
accomplished previously with emitting compounds that emit light in
the region of 640 nm to 740 nm. Clarity of the emission, however,
has been problematical because the light is emitted at wavelengths
which deviate significantly (i.e., <650 nm) from those
associated with saturated red. For example, emissions at about 675
nm are in the deep red region, while emissions somewhat below 650
nm generally result in a faded red emission. It is therefore
desirable to formulate an OLED which is capable of emitting
saturated red light with wavelengths in a region closely defined
around 650 nm. For comparison purposes, the CIE color coordinates
for a red video signal, according to the International
Telecommunications Union (ITU) would be x=0.6430; y=0.330.
In accordance with some of the embodiments of the present
invention, a dopant capable of shifting the emission wavelength of
an emissive layer comprised only of a host compound, is added to a
host compound in an amount effective to shift the wavelength of
emission so that the LED device emits light that is perceived by
the human eye to be close to a saturated red color. Although it is
recognized that characterization of color perception is a
subjective exercise, a quantitative chromaticity scale has been
developed by the Commission Internationale de l'Eclairage
(International Commission of Illumination), otherwise known as the
CIE standard. According to this standard, a saturated red color may
be represented by a single point, with specific quantitative
coordinates according to the defined axes of the chromaticity
scale. It will be appreciated by one of skill in the art that such
a single point on the CIE scale would represent a standard or a
goal that, in practical terms, is difficult, but fortunately,
unnecessary, to attain.
In the preferred embodiments of the present invention in which the
OLED produces a predominantly red emission, the dopants are
incorporated into a host compound so that the OLED emits light that
is perceived by the human eye to be close to a saturated red color.
Through the practice of the present invention, it is intended that
OLEDs be constructed which can be characterized by an emission that
is close to an absolute (or saturated) chromaticity value, as that
would be defined by the CIE scale. Furthermore, LED's utilizing the
materials of the present invention are also intended to be capable
of a display brightness that can be in excess of 100 cd/m.sup.2
although somewhat lower values, perhaps as low as 10 cd/m.sup.2,
may be acceptable in certain cases.
The host compounds as defined herein are compounds which can be
doped with dopants to emit light with the desired spectral
characteristics. Such compounds include, but are not limited to,
both emitting compounds and host compounds as described in U.S.
patent application Ser. No. 08/693,359, filed 6 Aug. 1996,
incorporated herein by reference. The term "host" is used to refer
to the compound in the emissive layer that functions as the
component which receives the hole/electron recombination energy and
then by an emission/absorption energy transfer process, transfers
that excitation energy to the dopant compound, which is typically
present in much lower concentrations. The dopant may then relax to
an excited state having a slightly lower energy level, which
preferentially radiates all of its energy as luminescent emission
in a desired spectral region. A dopant that radiates 100% of the
dopant's excited state excitation energy is said to have a quantum
efficiency of 100%. For host/dopant concentrations which are to be
used in a color tunable SOLED, preferably most, if not all, of the
host's excitation energy is transferred to the dopant which in turn
radiates, perhaps from a lower energy level, but with a high
quantum efficiency to produce visible radiation having a desired
chromaticity. The present invention is directed toward classes of
compounds that are intended to serve as hosts and/or dopants,
together or with still other hosts and/or dopants, so as to satisfy
these demanding energy transfer requirements.
As the term host compound is used herein, it will be appreciated
that such compounds can be found in either an electron
transporting/emissive layer of a single heterostructure OLED device
or in the separate emissive layer of a double heterostructure
device. As will be recognized by one of skill in the art, the use
of the host and dopant species such as disclosed herein makes it
possible to extend not only the range of colors emitted by the
OLED, but also to extend the range of possible candidate species
for host and/or dopant compounds. Accordingly, for effective
host/dopant systems, although the host compound can have a strong
emission in a region of the spectrum where the dopant species
strongly absorbs light, the host species preferably does not have
an emission band in a region where the dopant also emits strongly.
In structures where the host compound also functions as a charge
carrier, then additional criteria such as redox potential for the
species also becomes a consideration. In general, however, the
spectral characteristics of the host and dopant species are the
most important criteria.
The amount of dopant that is present is that amount which is
sufficient to shift the emission wavelength of the red emitting
material as close as possible to saturated red, as that would be
defined according to the CIE scale. Typically, the effective amount
is from about 0.01 to 10.0 mol %, based on the emitting layer. The
preferred amount is from about 0.1 to 1.0 mol %. The primary
criterion for determining an appropriate doping level is the level
which is effective for achieving an emission with the appropriate
spectral characteristics. By way of example, and without
limitation, if the amount of dopant species is at too low a level,
then emission from the device will also comprise a component of
light from the host compound itself, which will be at shorter
wavelengths than the desired emission form the dopant species. In
contrast, if the level of dopant is too high, emission efficiencies
could be adversely affected by self-quenching, a net non-emissive
mechanism. Alternatively, too high levels of the dopant species
could also adversely affect the hole or electron transporting
properties of the host material.
Thus, while there may be substantial variation in the type, number,
thickness and order of the layers that are present, dependent on
whether the device includes a single heterostructure or a double
heterostructure, whether the device is a stacked OLED or a single
OLED, whether an inverted sequence of OLED layers is present,
whether the OLED is intended to produce a predominantly red
emission, or whether still other design variations are used, the
present invention is directed to those devices in which the OLED is
comprised of a heterostructure for producing electroluminescence
which has an emissive layer containing at least one of the host
and/or dopant materials as disclosed herein. For example, one of
the embodiments of the present invention is directed to an emissive
layer containing a host material of a metal complex of
(5-hydroxy)quinoxaline, ##STR4## wherein M is Al, Ga, In, Zn or Mg,
with n=3, if M is Al, Ga or In and n=2, if M is Zn or Mg.
More specifically, one of the embodiments of the present invention
is directed to dopant materials comprised of an inner salt having
the chemical structure of formula III: ##STR5## wherein R.sub.1,
R.sub.2, R.sub.3 and R.sub.4 are, independently of each other, an
unsubstituted or substituted alkyl, aryl or heterocycle (for
example, a pyrole), and R.sub.5 and R.sub.6 are, independently of
each other, an unsubstituted or substituted alkyl, aryl, OH or
NH.sub.2. Such compounds are herein referred to as
bisphenyl-squarilium compounds.
Still more specifically, one of the embodiments of the present
invention is directed to dopant materials comprised of an inner
salt of a compound having the chemical structure of formula IV:
##STR6## wherein R=an alkyl. Such compounds are herein referred to
as squarilium dyes.
Another embodiment of the present invention is directed to dopant
materials comprised of an indigo dye compound having the chemical
structure of formula V: ##STR7## wherein X=NH, NR.sub.9, S, Se, Te,
or O, wherein R.sub.9 is alkyl or phenyl, and R.sub.7 and R.sub.8
are, independently of each other, an unsubstituted or substituted
alkyl or aryl group, or a .PI.-electron donor group such as --OR,
--Br, --NR.sub.2, etc. or a .PI.-electron acceptor group such as
--CN, --NO.sub.2, etc.
Still more specifically, another of the embodiments of the present
invention is directed to dopant materials comprised of an indigo
dye compound having the chemical structure of formula VI: ##STR8##
wherein X=NH.
Still another embodiment of the present invention is directed to
dopant materials comprised of a fullerene compound, for example,
the C.sub.60 fullerene compound.
Methods for preparing each of the disclosed dopant compounds are
known in the art.
As an illustrative example of the present invention, the host
compound, aluminumtris(5-hydroxy-quinoxaline), ("Alx.sub.3 ") is
preferably comprised of the compound of formula I wherein M=Al and
n=3: ##STR9## and the dopant compound is preferably comprised of
the squarilium dye compound of formula IVa,
1,3-bis[4-(dimethylamino)-2-hydroxyphenyl]-2,4-dihydroxycyclobuteneolyllum
dihydroxide,bis(inner salt)[63842-83-1]: ##STR10##
Vacuum-deposited, single heterostructural OLEDs have been made
having an electron transporting layer comprised of a host compound
of formula II and a dopant compound of formula IVa, wherein the
OLED has current-voltage (I-V) characteristics, UV-visible,
photoluminescence and electroluminescence properties that are
particularly suitable for OLEDs. It is believed that the
effectiveness of this particular host/dopant combination is based
on achieving a high level of energy transfer from the host to the
dopant. Such a high level of energy transfer, in some cases for
suitably matched hosts and dopants, can lead to more efficient
electroluminescence as compared to when the host compound is used
alone. While not intending to be limited by the theory of the
invention for the representative embodiments as disclosed herein,
as a means of illustrating how such combinations of host and dopant
compounds can be selected and matched to provide efficient
electroluminescence (EL), the photoluminescence (PL) of the host
Alx.sub.3 compound, with and without the dopant dye compound, may
be compared with Alq.sub.3 : ##STR11## which was also prepared with
and without the dopant compound.
The PL of these compounds may be measured using standard
techniques, for example, by immersing the compounds in a solvent,
exposing them to a photoexcitation source and measuring the
photoluminescence spectrum as a function of wavelength using
equipment such as is available from Photon Technology
International, in Somerville, N.J.
The PL spectrum of Alq.sub.3, Alx.sub.3 and Gax.sub.3 is shown in
FIG. 2. The Alx.sub.3 produces an orange photoluminescence with a
maximum at about 620 nm, which is significantly red shifted
relative to Alq.sub.3, which has a PL maximum at about 515 nm. It
is believed that this large red shift can be significant in helping
to achieve the high level of energy transfer from the host
Alx.sub.3 compound to the red fluorescent dye dopant. The EL
spectra of OLEDs prepared with an un-doped and doped Alx.sub.3
layer are shown in FIG. 3. Whereas the undoped host material has an
EL spectrum that is slightly red shifted as compared with the PL
spectrum of the host material, the maximum occurs in a wavelength
region that still has an orange appearance, as characterized, for
example, by the standard CIE calorimetric system (x=0.565,
y=0.426). However, when this host material is doped with the inner
salt dye of formula IVa, the EL spectrum has a maximum in a
wavelength region that is significantly shifted toward the red
(x=0.561, y=0.403).
Further embodiments of the present invention are directed to OLEDs
having an emissive layer containing a dopant comprised of an indigo
dye compound of formula V or of a fullerene compound. Without in
any way limiting the scope of these classes of compounds, species
of these classes of compounds are represented by the indigo dye
compound of formula VI and the fullerene compound C.sub.60. The
absorbance spectra of these compounds, as shown in FIG. 5, together
with the bisphenol squarilium compound of formula IVa, show that
these compounds have absorption bandwidths which are suitably
matched to accept the photoluminescence from the host compounds
Alx.sub.3 and Alq.sub.3.
The photoluminescent spectra of these dopants, as shown in FIG. 6,
show that each of these compounds produces luminescence in or
toward the red region of the visible spectrum. Furthermore, as
illustrated by the electroluminescent spectra of OLEDs
incorporating the fullerene or squarilium dye dopants in a single
heterostructural device using Alx.sub.3 or Alq.sub.3 as the host
material, each of these compounds is shown to be capable, at
sufficient dopant concentrations, of having the excitation energy
completely transferred from the host, see FIGS. 7, 8 and 9. Part of
that energy is then radiated as electroluminescence by the dopant.
The capacity for a dopant to replace substantially all of the
emission of the host compound with that of the dopant is of
particular benefit, for example, in a color tunable SOLED. In such
devices, which have more than one color-emitting layer, it is
desirable to have each layer with its own well-defined chromaticity
and with a spectral emission that does not overlap the spectral
emission of any other layer.
The electroluminescent emission of an OLED containing the
representative indigo dye compound is shown to have an emission
band having a maximum near 650 nm (with CIE values, x=0.693 and
y=0.305), which produces a saturated red appearance.
The Zn and Mg derivatives of (5-hydroxy)quinoxaline have also been
prepared and found to produce PL spectra nearly identical to that
observed for Alx.sub.3. In view of the effectiveness of Alx.sub.3
as a host material for red-emitting dopants wherein there is also a
good energy match, such PL spectra show that the Zn and Mg
derivatives may also be useful as host materials for suitably
selected red-emitting dopants.
The Ga derivative of (5-hydroxy)quinoxaline has also been prepared
and found to have the PL emission spectrum as shown in FIG. 2.
These results show that the Ga analog may be effective not only as
a doped emissive material, but also as an un-doped material.
The OLEDs may be prepared using the materials, techniques and
apparatus such as described, for example, in co-pending
applications, "High Reliability, High Efficiency, Integratable
Organic Light Emitting Devices and Methods of Producing Same," Ser.
No. 08/774,119; "Novel Materials for Multicolor LED's," Ser. No.
08/771,815; "Electron Transporting and Light Emitting Layers Based
on Organic Free Radicals," Ser. No. 08/774,120; "Multicolor Display
Devices," Ser. No. 08/772,333; and "Red-Emitting Organic Light
Emitting Devices (LED's)," Ser. No. 08/774,087, each of said
co-pending applications being filed on Dec. 23, 1996; "Vacuum
Deposited, Non-Polymeric Flexible Organic Light Emitting Devices,"
Ser. No. 08/789,319, said co-pending application being filed on
Jan. 23, 1997; "Driving Circuit for Stacked Organic Light Emitting
Devices," Ser. No. 08/792,050; "Displays Having Mesa Pixel
Configuration," Ser. No. 08/794,595; and "Stacked Organic Light
Emitting Devices," Ser. No. 08/792,046, each of said co-pending
applications being filed on Feb. 3, 1997. The present invention as
disclosed herein may be used in conjunction with the subject matter
of any one or more of these co-pending application, which are each
incorporated herein in their entirety by reference. The present
invention may also be used in conjunction with the subject matter
of each of co-pending U.S. patent applications Ser. Nos.
08/354,674, 08/613,207, 08/632,316, 08/632,322 and 08/693,359 and
U.S. provisional patent applications Ser. Nos. 60/010,013 and
60/024,001, each of which is also herein incorporated in its
entirety by reference.
This invention will now be described in detail with respect to
showing how certain specific representative embodiments thereof
will be made, the materials, apparatus and process steps being
understood as examples that are intended to be illustrative only.
In particular, the invention is not intended to be limited to the
methods, materials, conditions, process parameters, apparatus and
the like specifically recited herein.
EXAMPLES
OLEDs were prepared by successively depositing the OLED layers on a
commercially available borosilicate glass substrate that was
precoated with ITO.
In these particular embodiments, in which the host/dopant materials
of the present invention were prepared and incorporated into an
OLED, the hole transport layer was comprised of
N,N'-diphenyl-N,N'-bis(3-methylpheny)1-1'biphenyl-4,4'diamine
(TPD), and the electron transporting layer was comprised of the
doped or undoped tris-(5-hydroxyquinoxaline) aluminum (Alx.sub.3)
or of doped or undoped Alq.sub.3.
The hole transporting material TPD and the electron transporting
materials Alq.sub.3 and Alx.sub.3 were synthesized according to
literature procedures, and were sublimed before use.
The dopant C.sub.60 was purchased from Southern Chemical Group,
LLC, and used as received.
The bisphenol-squarilium dopant of formula IVa and the indigo dye
compound dopant were purchased from Aldrich and sublimed twice to
give the pure green and purple crystalline materials,
respectively.
Aluminum tris (5-hydroxyquinoxaline), (Alx.sub.3), was prepared by
the reaction of aluminum trisisopropoxide with 5-hydroxyquinoxaline
in isopropanol under argon. Isopropanol was dried over CaH.sub.2
before use. The amount of the ligand taken for the reaction was in
slight excess. The isopropanol mixture was refluxed under argon for
one and a half hours, and orange product was isolated by
rotarvaporization.
Gallium tris(5-hydroxyquinoxaline), (Gax.sub.3), was formed by
mixing 200 ml water solution of 0.2 g Ga(NO.sub.3).sub.3
.cndot.xH.sub.2 O and an excess of 1% alcoholic solution of the
ligand at 60.degree. C., followed by 10% ammonium hydroxide
addition to make the solution slightly basic. Orange precipitate
was obtained, and, after cooling, filtered off. The Gax.sub.3
preparation was similar to that described in Spectrochimica Acta,
1956, Vol. 8, pp. 1-8.
For the reagents, 5-hydroxyquinoxaline was prepared according to S.
K. Freeman, P. E. Spoerri, J. Org. Chem., 1951, 16, 438; aluminum
trisisopropoxide (99.99% purity) and Ga(NO.sub.3).sub.3
.cndot.xH.sub.2 O (99.999% purity) were bought from Aldrich; and
isopropanol was obtained from Fisher Scientific.
The ITO/Borosilicate substrates (100.OMEGA./square) were cleaned by
sonicating with detergent for five minutes followed by rinsing with
deionized water. They were then treated twice in boiling
1,1,1-trichloroethane for two minutes. The substrates were then
sonicated twice with acetone for two minutes and twice with
methanol for two minutes.
The background pressure prior to deposition was normally
7.times.10.sup.-7 torr or lower and the pressure during the
deposition was about 5.times.10.sup.-7 to 1.1.times.10.sup.-6
torr.
All the chemicals were resistively heated in various tantalum
boats. TPD was first deposited at a rate from one to four .ANG./s.
The thickness was typically controlled at about 300 .ANG..
The electron transporting layer (Alq.sub.3, Alx.sub.3) was doped
with various dyes (C.sub.60, the bisphenol squarilium dye and the
indigo dye compound of formula VI). Typically, the dopant was first
vaporized with the substrates covered. After the rate of the dopant
was stabilized, the host material was vaporized at the prescribed
rate. The cover over the substrates was then opened and the host
and guest were deposited at the desired concentration. The rate of
dopant was normally 0.1-0.2 .ANG./s. The total thickness of this
layer was controlled at about 450 .ANG..
The substrates were then released to air and masks were put
directly on the substrates. The masks are made of stainless steel
sheet and contain holes with diameters of 0.25, 0.5, 0.75, and 1.0
mm. The substrates were then put back into vacuum for further
coating.
Magnesium and silver were co-deposited at a rate normally of 2.6
.ANG./s. The ratio of Mg:Ag varied from 7:1 to 12:1. The thickness
of this layer was typically about 500 .ANG.. Finally, 1000 A Ag was
deposited at the rate between one to four .ANG./s.
The I-V characteristics of these OLEDs were measured and are shown
for the doped and undoped Alx.sub.3 in FIG. 4. The close similarity
of the results with and without the dopant shows that the dopant is
not perturbing the electrical properties of the device.
For the OLEDs containing an emissive electron transporting layer of
Alq.sub.3 as the host material with and without the same
fluorescent dye dopant, wherein the energy match between the host
material and the dopant was poor, nearly all of the EL emission was
obtained from the Alq.sub.3.
Those of skill in the art may recognize certain modifications to
the various embodiments of the invention, which modifications are
meant to be covered by the spirit and scope of the appended
claims.
* * * * *